Around 44,000 people in the United States are diagnosed with acute leukemia each year and 22,000 people die from the disease. Rearrangements of the human Mixed Lineage Leukemia (MLL) gene are one of the most common abnormalities in acute leukemia and are a hallmark for aggressive (high-risk) pediatric, adult and topoisomerase ll-associated cases. Understanding the mechanisms of transformation by MLL fusion proteins will be useful in developing targeted therapy for these patients. MLL is a homologue of Drosophila Trithorax (Trx), which is involved in the transcriptional activation of the target genes. This activity is mediated in part through histone 3 lysine 4 (H3K4) methylation. However, with MLL translocation, the C-terminal SET domain responsible for this enzymatic activity is lost, and the truncated. MLL is fused in frame to one of more than fifty translocation partners. The most common MLL translocation partners are AF4, ENL, AF9 and AF5q31, which accounts for more than 70% of MLL-associated leukemias. These four translocation partners have recently been shown to directly interact in a complex called MPAC (MLL fusion Partner Associated Complex), along with histone 3 lysine 79 methyltransferase Dot1L, RNA polymerase II C-terminal domain kinase pTEFb, and Polycomb group proteins Pc3 and Ring1b. This proposal will study how these diverse proteins interact with each other to promote leukemogenesis. Specific Aim 1 will address the role of MPAC in transcriptional activation and transformation, and Specific Aim 2 will assess the regulation of MPAC subunit interaction. Study of the MPAC activity will provide valuable insights into how normal transcriptional mechanisms are disrupted by MLL fusion proteins and how these may be targeted therapeutically.